This lecture is divided into hyperlinked sections
Introduction
What is the
function of a web browser?
More Browser Functions
Extra Features
of Browsers
MIME
Browser Choices
HTML
Conclusion
Resources
Tutorial Questions
This section of the course is concerned with web browsers.
If you are reading this then you are using a web browser. Statistically
you are likely to be using Internet Explorer as provided by Microsoft,
otherwise you are probably using Netscape Navigator as provided by Netscape
Communications Corporation.
A web browser is a piece of software that resides on your computer that allows you to read and view web related content. When you surf the Internet for information etc. you visit web sites for content.
The simplest web pages that are available are written in a language known as hypertext markup language or HTML. The function of HTML is to provide a format in which text and pictures and hyperlinks can be described. It contains standard codes (or tags) that determine how a Web page looks when your browser displays it.
By describing the elements that go to make up a web page, the attributes of the text (e.g. size, font, positioning etc.) can be defined in such a way that the definition can be transmitted via the Internet to your computer and passed through your web browser. The browser looks at the definition and renders (displays) the text, pictures etc. in the place that the author intended.
For an example of HTML, click here.
A browser can also interpret the commands that may be present in a web page. Among the scripting languages currently in use are JavaScript, JScript and VBScript. These scripting languages are able to control the way your browser operates and can take input from the user or the user's system in order to return results that are base don the input.
Scripting languages allow some flexibility in rendering the web pages. Pure HTML produces a static web page that can only be viewed or used to access another page via a hyperlink. The WWW has brought about a change in the way that information is presented to the end-user. Unlike television, which is a push technology (simplex) the WWW allows for pull content in which the user is able to choose what he/ she sees and how the content is arranged on the viewing screen.
JavaScript grew from LiveScript which was written by Netscape to accompany their 2.0 release of their web browser. This added extra functionality to the browser and web pages that it displayed. Not to be left out, Microsoft decided that they would define their own scripting language and produced JScript, which borrows much from C++. Microsoft also were responsible for introducing VBScript, somewhat easier to learn than JavaScript.
Unfortunately for Microsoft, Netscape Navigator does not support VBScript without a plug-in and so the content changes offered by VBScript will not be seen in a Navigator browser without a plug-in.
Internet Explorer does support JavaScript however and many website developers have realised this. The consequence is that JavaScript is more widely used and supported than the Microsoft scripting options.
Apart from requesting and displaying web content, the browser is turning into a multi-purpose tool on the desktop. Once you have downloaded a page, it is possible to use a tool within the browser to visit the previous page or alternatively use storage space within the browser to create and visit useful pages that the user has defined as Bookmarks or Favorites. Some browsers also provide a history of the pages that you have visited over a user-defined period so that it will be easy to find a site that you have forgotten the address of.
The browsers also can provide a search function, but that is not an internal service, it calls upon the services of an external server to perform that function.
A 'Find' feature is built into the browser so that if you have searched for a specific word using a search engine and have downloaded a document containing that word, you can search easily for it.
Other features available are toolbars at the top of the
screen to provide a 'one click' solution to a common task such as reloading
the page from the server, integrated email (Netscape), integrated HTML
editor (Netscape) and many other functions.
MIME (Multimedia Internet Mail Extensions)
Apart from displaying pages written in HTML, a browser is also able to display or present to the user other data types such as pictures, music (mp3), non-HTML documents e.g. pdf, and other content not specifically designed to be displayed using a browser.
This is achieved because the server is able to indicate to the client the type of content that is about to be transferred. The type of content is indicated using MIME extensions. MIME types consist of a main type and a subtype, e.g. 'image/ jpeg' specifies the type as an image and the storage format being jpeg (Joint Photographic Expert Group).
When the browser has been informed of the type of content about to arrive it can make a choice of the most appropriate way of processing the content. The browser may handle the content internally or alternatively call upon the services of of an external program.
An example of this is when you listen to streamed radio content from the BBC. The browser, Netscape Navigator or Internet Explorer, invokes the services of an external application (Real Player). This is because that particular type of content (streamed audio) that is sent to your computer via the Internet cannot be displayed or rendered using the browser itself. Thus it calls upon the services of an external program.
There are two competing browser technologies. One is Netscape Navigator and the other is Internet Explorer from Microsoft. There are other browsers available such as Opera which are trying to break into the browser market.
Older versions of browsers exist such as Lynx which is a text only browser and Mosaic which is an early graphical browser.
This is Hypertext Markup Language and is the original language used for producing web pages. HTML documents are passed through the Internet using a protocol (set of rules) known as Hypertext Transfer Protocol (HTTP).
The reason for HTML's existence is that it is platform and application independent. Thus it can describe to any client the way in which a page should be displayed. The current version of HTML is version 4.01. It has been defined by the World Wide Web Consortium.
To define a language such as HTML, two separate specifications are required, SGML (Standard Generalised Markup Language) which is a language definition and DTD (Document Type Definition).
SGML cannot be used to write a page, it merely defines scope such as character set(s) size limits etc. SGML was defined in 1986 as ISO 8879:1986 and its purpose was to unify the markup languages that were in use up to that point such as TeX and Postscript. It defines how the components of the language interact with each other
A DTD defines the items that will be used within the language such as the tags that will be legal.
A web browser has a basic function to request and display web pages and their content. The browser market is keenly competed for by Netscape and Microsoft.
MIME is used to describe the web content to the browser so that it can make the appropriate choice to display the content.
HTML 4.01 is the current standard for writing web pages.
Browsers can also act upon the commands issued by scripting
languages. Netscape's JavaScript looks likely to be the most commonly used
scripting language. Scripting languages provide a means of making pages
have more active content then pure HTML which has no active content.
Read the documentation
published by Microsoft to accompany its browser.
Read the documentation
published by Netscape to accompany its browser.
MIME
documentation and associated RFCs
W3C World Wide Web Consortium
- responsible for developing interoperable technologies for the Internet
HTML
4.01 DTD
Is it a good idea to have a browser that is fully integrated into the operating system of the computer, such as Internet Explorer and Windows XX?
What would the benefits of such a system be?
What would the drawbacks of such a system be?
What MIME types can you think of that can be dealt with by the browser?
What MIME types can you think of that require an external
application to display/ play them?
(c) MM Clements 2001 Back to top of Page